Cervical cancer in humans has been shown to be caused by a virus (HPV). It seems to me that it is likely that these other 'contagious cancers' are caused by, or at least require, viruses or other pathogens that have not yet been identified. So, it's not that the cancer is contagious. Rather it is that the pathogen that triggers the cancer (or stops the body from effectively dealing with cancerous cells) is.

But I could well be wrong. Perhaps what those Tasmanian devils really need is to calm down with a bit of mindfulness training and to eat more vegetables.

Edit: I probably am wrong. Seems researchers think the Tasmanian devil cancer is caused by the transmission of the cancer cells themselves (allograft). And hamsters have a contagious cancer too.
From Wikipedia Devil facial tumour disease

Cervical cancer in humans has been shown to be caused by a virus (HPV). It seems to me that it is likely that these other 'contagious cancers' are caused by, or at least require, viruses or other pathogens that have not yet been identified. So, it's not that the cancer is contagious. Rather it is that the pathogen that triggers the cancer (or stops the body from effectively dealing with cancerous cells) is.

"In the mid-1990s, the first signs were observed of the fatal and infectious cancer, Devil Facial Tumour Disease. Sightings of the Tasmanian devil have since declined by more than 70 per cent."

There are fears that the Tasmanian devil could end up extinct due to this. Scientists havent been able to figure it out.

This makes one wonder if a similar thing could easily happen to us humans.. a fatal infectious cancer could suddenly appear out of the blue like it did with the Tassy devils and within 25 years wipe out 70% of us (or who knows it could be even more infectious or dangerous). Kinds of shows how little us humans do know and just how vulnerable we could be to all kinds of deadly illnesses suddenly appearing in us. I dont think its ever been considered about possibly an infectious cancer wiping us all out and some cancers in us still have hugely high depth rates eg pancreatic cancer I think is very high.